There is considerable interest in developing predictive capabilities for social diffusion processes, for instance enabling early identification of contentious “triggering” incidents that are likely to grow into large, self-sustaining mobilization events. Recently we have shown, using theoretical analysis, that the dynamics of social diffusion may depend crucially upon the interactions of social network communities, that is, densely connected groupings of individuals which have only relatively few links to other groups. This paper presents an empirical investigation of two hypotheses which follow from this finding: 1.) the presence of even just a few inter-community links can make diffusion activity in one community a significant predictor of activity in otherwise disparate communities and 2.) very early dispersion of a diffusion process across network communities is a reliable early indicator that the diffusion will ultimately involve a substantial number of individuals. We explore these hypotheses with case studies involving emergence of the Swedish Social Democratic Party at the turn of the 20th century, the spread of SARS in 2002–2003, and blogging dynamics associated with potentially incendiary real world occurrences. These empirical studies demonstrate that network community-based diffusion metrics do indeed possess predictive power, and in fact can be significantly more predictive than standard measures.